The present invention relates to improved thermal sprayable powders, particularly molybdenum-iron alloy powders suitable for forming improved coatings on metal substrates having high thermal conductivity and wear resistance. More particularly, the present invention relates to improved molybdenum-iron alloy powders suitable for thermal spraying and forming corrosion and wear resistant coatings having good thermal conductivity.
Surfaces subject to wear at elevated temperatures are often coated with metal alloys to reduce wear and to provide improved conductivity. For example, yankee dryer rolls used by the paper and pulp industries are often faced with a coating comprising 75% by weight molybdenum and 25% by weight nickel alloy. The coating is typically formed using a plasma spray gun into which is fed a blend of molybdenum and alloy powders. Yankee dryer rolls may exceed 20 feet in diameter and are often 30-40 feet in length. The molybdenum-nickel coatings are, however, susceptible to corrosion, reducing the useful life of such coatings, particularly in the corrosive environment to which yankee dryer rolls are subjected.
In such coatings, molybdenum provides improved wear resistance, high thermal conductivity and functions as a tribological couple with the doctor blade. These properties are critical in such applications, making molybdenum a preferred material for such coatings. Nickel alloy is added primarily to serve as a binder to hold the molybdenum particles together in the coating. Unfortunately, however, such nickel alloys have relatively poor thermal conductivity and when nickel is in contact with molybdenum under the service conditions of a yankee dryer rolls, galvanic potential exacerbates corrosion. As will be understood, corroded surfaces on yankee dryer rolls are unacceptable, requiring replacement or refacing, which is an expensive, time consuming procedure, particularly given the size of the rolls. A corrosion resistant coating is thus needed for such applications, which must also have good thermal conductivity and wear resistance.
As will be understood by those skilled in the art, there are many other applications requiring improved wear resistance and good thermal conductivity which do not necessarily require corrosion resistance. For example, there are numerous applications for coatings having improved wear resistance and good thermal conductivity in the automotive and aerospace industries. The coatings of this invention may be used for such applications as piston rings and shifter forks, for example.
Another problem with present wear resistant coatings requiring good thermal conductivity and wear resistance is the method of application. Where the constituents of the coating alloy must be fed as a blend of separate metal or metal alloy powders, the consistency of the resultant coating alloy may be adversely affected. Alloy thermal spray coating powders are, however, limited to alloys which may be formed by conventional atomization techniques. That is, the alloy formulation must be capable of being melted and atomized. Also, the alloy metal powder must be suitable for thermal spray applications, preferably suitable for both plasma and HVOF (high velocity oxygen flame) thermal spray apparatus. Thus, there is a need for a thermal sprayable metal alloy powder which may be used to form improved wear and abrasion resistant coatings having high thermal conductivity and most preferably coatings which are also corrosion and oxidation resistant. The improved thermal sprayable molybdenum-iron alloy powder of this invention meets these criteria.